How to increase transmission system capacity and stability

Series compensation (SC)

The ongoing transformation of the energy system requires effective means to cope with growing challenges when it comes to reliable power transmission. Instead of building new lines to provide sufficient transmission capacity, it’s often better to use our flexible AC transmission systems (FACTS) technology to increase the transmission capacity. Both newly built transmission lines and existing systems benefit from Siemens’ advanced series compensation (SC) solutions that also help to improve the quality and stability of power transmission lines.

Series compensation at a glance

Boost your transmission lines

Innovative series compensation technology provides higher transmission capacities for existing long-distance AC transmission lines and increased grid stability without the costs and time-requirement of building new lines.

For decades, series compensation (SC) technology from Siemens has improved grid stability and contributed to the optimal utilization of transmission lines. Transmission system operators around the world have already benefited from solutions that were tailored to their specific requirements. Among the main benefits of series compensation are the reduction of line voltage drops, limitation of load-dependent drops, and a reduction of the transmission angle.

From fixed capacitors to high-power thyristors

Series compensation comes in three variations that offer different advantages and features. Our portfolio comprises the proven, simple fixed series capacitor and thyristor-equipped capacitors for advanced capabilities.

Cost-effective capacity expansion

The amount of power that can be transferred with long overhead transmission lines is limited by the impedance that can lead to voltage drops. For decades, fixed series compensation is the proven solution to maintain a minimum voltage profile and maximize utilization of transmission lines. 

It works by connecting a capacitor bank in series with the transmission line to partially compensate the inductive impedance of the line while also increasing the voltage at the point of connection. The capacitors are protected by metal-oxide varistors (MOV) and – in case of a fault – by tripping the FSC and closing the bypass breaker. For instant protection, a triggered spark gap bypasses capacitors and MOVs within less than 1 ms. Even gap-less solutions are possible, provided that local requirements match. Due to this complexity Siemens recommends and offers individually designed solutions with fixed series capacitors.

Fastest solution for service restoration

When maximum transmission capacity and availability are essential, a thyristor-protected series capacitor (TPSC) provides significant advantages over a fixed series capacitor. In combination with a current-limiting reactor, it protects the capacitor bank from overvoltages and fault currents, and allows immediate line reinsertion. As this solution doesn’t require MOVs and spark-gap, there’s no cool-down time after a fault. 

Instead, in case of a fault the high-power thyristors are fired to bypass the capacitors within milliseconds. The air-cooled thyristors allow very quick reinsertion of the line, whereas cool-down times of MOVs can last minutes or hours, depending on the absorbed fault energy. This makes TPSC the preferred series compensation solution when maximum availability is key.

Dynamic line impedance control

To dynamically control the line impedance, a thyristor-controlled series capacitor (TCSC) is the best solution of choice. While fixed series capacitors only allow one discrete step in changing the line impedance, a TCSC allows impedance control within its designed control window. 

By actively changing the impedance it becomes possible to damp power oscillations (POD) and to mitigate sub-synchronous resonance (SSR). Both are important for the stability and safety of generators in the grid, which could be harmed by the interaction of eigenfrequencies with the resonance frequency of the compensated line. The TCSC also enables specific line compensation when operating in steady state.

Dynamic control

Actively controlling impedance

Series compensation provides the proven tools for controlling a line’s impedance in order to increase transmission capacity and grid stability. With thyristor-equipped capacitors it’s even possible to actively and dynamically control the impedance. Individually designed solutions provide the maximum outcome for grid operators.